The production of 6-hydroxy-2-naphthoic acid, an intermediate useful for the preparation of synthetic fibers, and structural plastics, by the reaction of the potassium salt of 2-hydroxynaphthalene and carbon dioxide at an elevated temperature, is disclosed by Andre in U.S. Pat. No. 1,593,816. Andre, however, does not disclose the criticality of temperature, pressure, or ratios of reactants. Moreover, upon repeating Andre's work, the actual yield of 6-hydroxy-2-naphthoic acid obtained was found to be only about one-third of the yield reported by Andre. Additionally, an improved process is disclosed and claimed in pending application Ser. No. 127,703, filed Mar. 6, 1980, now U.S. Pat. No. 4,287,357 herebyincorporated herein by reference.
The carboxylation of alkali metal salts of phenols with carbon dioxide to form acids containing phenolic substituents, the well-known Kolbe-Schmitt reaction, is also disclosed in U.S. Pat. Nos. 3,816,521 and 3,405,169.
In the carboxylation of the potassium salt of 2-hydroxynaphthalene, the initial product formed is 3-hydroxy-2-naphthoic acid which subsequently rearranges in situ to form 6-hydroxy-2-naphthoic acid. This rearrangement is rapid until the ratio of 6-hydroxy-2-naphthoic acid to 6-hydroxy-2-naphthoic acid plus 3-hydroxy-2-naphthoic acid is approximately about 0.6-0.8 and the rearrangement is slow up to the equilibrium ratio of about 0.9-0.93. This slow rearrangement requires a long reaction time which leads to undesirable tar formation and, accordingly, lost yield, increased energy use and increased cost. The 3-hydroxy-2-naphthoic acid which is capable of recovery from the equilibrium product is contaminated with small quantities of 6-hydroxy-2-naphthoic acid and 2-naphthol dicarboxylic acid. It is of poor quality without extremely expensive reworking thereof and is thereforegenerally discarded.
There is need, therefore, for an improved process for the production of 6-hydroxy-2-naphthoic acid which will enable lower reaction times, increased yields based on 2-hydroxy naphthalene and reduced amounts of process residue, which, if discarded, must be done so in a non-polluting manner.